If it is happening because your camera is zoomed out far, here's a fix (i posted this on the sc2 forums a while ago):

I've found a way to fix this problem! I was having the same issue that you guys were talking about. In my map, I have the camera zoom out to a distance of 50, the default I think is around 35.

Here's the fix:

1. In the data editor, go to Data Type: Game UI Data.

2. Go to Default SC2 UI Settings.

3. Scroll down to Sound Data +, double click.

4. There is a list of about 20 different sound types. Scroll to somewhere in the middle and find Combat.

5. Once Combat is selected, scroll down to the very bottom, to the section Volume Rolloff Points.

6. For Index 1, change the Distance from 30 to some higher value. This value depends on how far out your camera is. Since I zoomed my camera out to 50, I figured that doubling this number from 30 to 60 would be enough. You may have to make yours higher than 60 depending on how zoomed out you are.

7. Repeat step #6 for other Sound types as needed. I changed types Death, Spell, UI, Build, Movement, and Other so far.

I haven't tested this out completely, but it seems to do the trick for now. Changing the volume instead of the distance may be a better way to fix it, or possibly adding another index, but I haven't tried that. I'm worried that I might pick up sounds far away in other areas of the map that I don't want, but so far it is working well.